Image pickup apparatus and image pickup system

ABSTRACT

In a photoelectric conversion apparatus that adds signals of a plurality of photoelectric conversion elements included in photoelectric conversion units, each of the plurality of photoelectric conversion elements includes a first semiconductor region of a first conductivity type that collects signal carriers. The first semiconductor regions included in photoelectric conversion elements that are included in each of the photoelectric conversion units and that are arranged adjacent to each other sandwich a second semiconductor region of a second conductivity type. A height of a potential barrier for the signal carriers generated in a certain region of the second semiconductor region is smaller than a height of a potential barrier for the signal carriers generated in a third semiconductor region between each of the first semiconductor regions and an overflow drain region of the first conductivity type.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One disclosed aspect of the embodiments relates to an image pickupapparatus and, more specifically, to isolation structure betweenphotoelectric conversion elements.

2. Description of the Related Art

Currently, in a photoelectric conversion apparatus, there is a case inwhich signals generated in a plurality of photoelectric conversionelements are processed as a signal of a single pixel. For example, atechnology has been disclosed in which focus detection using a phasedifference method is performed by focusing light onto a plurality ofphotoelectric conversion elements using a single microlens. For example,as described in Japanese Patent Laid-Open No. 2001-250931, focusdetection is performed by separately reading signals of photoelectricconversion elements corresponding to a single microlens. Thereafter, byadding the signals of the photoelectric conversion elementscorresponding to the single microlens, the signals may be treated as asignal of a single pixel.

When signals of a plurality of photoelectric conversion elements aretreated as a signal of a single pixel, an appropriate signal might notbe obtained if there are differences in sensitivity or the amount ofincident light between the plurality of photoelectric conversionelements. Especially because the photoelectric conversion elements arelocated adjacent to various elements, an appropriate signal might not beobtained depending on isolation structure between the adjacent elements.This is a process that may be performed not only in an application suchas phase difference detection but also in various applications, andgreater problems may arise as the application field of image pickupapparatuses develops.

The present embodiments provide a desired signal when a single signal isobtained using signals of a plurality of photoelectric conversionelements by making isolation structure between the photoelectricconversion elements and elements adjacent to the photoelectricconversion elements appropriate.

SUMMARY OF THE INVENTION

One of the embodiments is an image pickup apparatus including aplurality of photoelectric conversion units, each including a pluralityof photoelectric conversion elements. Signals of the plurality ofphotoelectric conversion elements included in the plurality ofphotoelectric conversion units are added. Each of the plurality ofphotoelectric conversion elements includes a first semiconductor regionof a first conductivity type that collects the signal carriers. Thefirst semiconductor regions included in photoelectric conversionelements that are included in each of the photoelectric conversion unitsand that are arranged adjacent to each other sandwich a secondsemiconductor region of a second conductivity type. A height of apotential barrier for the signal carriers generated in a certain regionof the second semiconductor region is smaller than a height of apotential barrier for the signal carriers generated in a region betweeneach of the first semiconductor regions and an overflow drain region ofthe first conductivity type.

Further features of the disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating the overall configurationof an image pickup apparatus according to each of the presentembodiments.

FIG. 2 is a diagram illustrating the schematic configuration of pixelsin the image pickup apparatus according to a first embodiment.

FIGS. 3A to 3D are diagrams schematically illustrating thecross-sectional structures and the minimum potentials of the pixels inthe image pickup apparatus according to the first embodiment.

FIG. 4 is a diagram schematically illustrating the output of the imagepickup apparatus according to the first embodiment.

FIGS. 5A to 5D are diagrams schematically illustrating the minimumpotential of the image pickup apparatus according to the firstembodiment.

FIGS. 6A and 6B are diagrams schematically illustrating thecross-sectional structure and the minimum potential of pixels in theimage pickup apparatus according to a second embodiment.

FIG. 7 is a diagram illustrating the image pickup apparatus according toa third embodiment viewed in plan.

FIG. 8 is a diagram schematically illustrating the cross-sectionalstructure of pixels in the image pickup apparatus according to a fourthembodiment.

FIG. 9 is a diagram schematically illustrating the imaging relationshipof an object.

FIGS. 10A and 10B are diagrams schematically illustrating focusdetection using a phase difference method.

FIG. 11 is a diagram illustrating the outline of an image pickupapparatus.

FIGS. 12A and 12B are diagrams schematically illustrating thecross-sectional structure and the minimum potential of pixels in animage pickup apparatus.

FIG. 13 is a diagram schematically illustrating the output of the imagepickup apparatus.

FIGS. 14A and 14B are diagrams illustrating examples of the equivalentcircuit of a photoelectric conversion unit.

DESCRIPTION OF THE EMBODIMENTS

One disclosed feature of the embodiments may be described as a processwhich is usually depicted as a timing diagram. A timing diagram mayillustrate the timing relationships of several entities, such assignals, events, etc. Although a timing diagram may describe theoperations as a sequential process, some operations may be performed inparallel or concurrently. In addition, unless specifically stated, theorder of the operations or timing instants may be re-arranged.Furthermore, the timing or temporal distances may not be scaled ordepict the timing relationships in exact proportions.

First, FIGS. 12A and 12B illustrate an example to facilitateunderstanding of the embodiments. FIG. 12A is a diagram illustrating thecross-sectional structure of pixels in an image pickup apparatus, andFIG. 12B in a lower part is a diagram schematically illustrating thepotential of a semiconductor layer.

Microlenses 1201 and color filters 1202 are illustrated. Wires 1203drive transistors in the pixels and supply power supply voltage and aground potential. A p-type semiconductor region 1205 is provided on ann-type semiconductor substrate 1204. N-type semiconductor regions 1206,1207, and 1211 are provided in such a way as to configure p-n junctionswith the p-type semiconductor region 1205. These regions configurephotodiodes (PDs in the following description) that serve asphotoelectric conversion elements.

A p-type semiconductor region 1208 is provided between the n-typesemiconductor regions 1207 and 1211 of the adjacent pixels that do notshare the same microlens 1201 and color filter 1202. Each p-typesemiconductor region 1208 may function as a potential barrier forelectrons, which are signal carriers, between the n-type semiconductorregions 1207 and 1211.

A p-type semiconductor region 1209 is provided between the n-typesemiconductor regions 1206 and 1207 that share the same microlens 1201.Each p-type semiconductor region 1209 may function as a potentialbarrier for electrons, which are signal carriers, between the n-typesemiconductor regions 1206 and 1207. The height of the potential barrierof the p-type semiconductor region 1208 is represented by a height 1210of the potential barrier illustrated in FIG. 12B.

In the case of such structure, when one of adjacent PDs has beensaturated due to a difference in sensitivity or luminance between thePDs, charge generated thereafter may leak. Moreover, charge generated inthe saturated PD may leak into the n-type semiconductor substrate 1204.Furthermore, the charge in the PD might leak into a floating diffusionregion configured by an n-type semiconductor region that serves as areading region to which the charge is transferred. Especially becausethe n-type semiconductor substrate 1204 and the floating diffusionregion may function as overflow drain regions (OFD regions) and the OFDregions originally aim to discharge saturated charge, the charge ishighly likely to leak into the OFD regions.

As a result of examinations conducted by the inventors, it has beenfound out that if a large amount of charge generated after a PD issaturated has leaked into an OFD region, a phenomenon illustrated inFIG. 13 may occur when signals of PDs are added and used for a capturedimage.

FIG. 13 illustrates a combined input/output characteristic at a timewhen the input/output characteristics of PDs (hereinafter referred to asa PD 1206 and a PD 1207) corresponding to the two n-type semiconductorregions 1206 and 1207 and the outputs of the PDs 1206 and 1207 arecombined. When light is incident on the PDs, photoelectric conversion isperformed and electron-hole pairs are generated. For the sake ofexplanation, FIG. 13 schematically illustrates a situation in which thePD 1206 has higher sensitivity than the PD 1207 or the amount of lightincident on the PD 1206 is larger than that incident on the PD 1207.When the amount of light incident on the PDs is within a range 1301illustrated in FIG. 13, the amount of charge generated is larger in thePD 1206 than in the PD 1207, but since the PD 1206 has not beensaturated, an output obtained by combining the signals of the PD 1206and the PD 1207 is appropriate. However, in the case of a range 1302illustrated in FIG. 13, the PD 1206 has been saturated, but the PD 1207has not been saturated. In this case, since the output of the PD 1206has been saturated, the output of the PD 1206 no longer becomes larger,whereas the PD 1207 has an appropriate output according to the incidentlight since the PD 1207 has not been saturated. Therefore, a combinedoutput 1307 after the PD 1206 is saturated is determined by a valueobtained by combining the outputs of the PD 1206 and the PD 1207, and,as a result, the combined output may have a knee characteristic afterthe PD 1206 is saturated. This phenomenon is significant when the chargegenerated after the PD 1206 is saturated is leaking into regions otherthan the PD 1207, especially into OFD regions provided around the n-typesemiconductor regions that configure the PDs.

On the other hand, one of the present embodiments has a characteristicthat the height of a potential barrier provided between a plurality ofphotoelectric conversion elements included in the same photoelectricconversion unit is lower than the height of a potential barrier providedbetween an n-type semiconductor region and an OFD region of aphotoelectric conversion element.

Next, a block diagram of a photoelectric conversion apparatus accordingto each of the present embodiments will be described. Here, an imagepickup apparatus is illustrated as an example of the photoelectricconversion apparatus. The embodiments may be applied to an apparatusother than the image pickup apparatus insofar as the apparatus utilizesphotoelectric conversion.

FIG. 1 is a diagram illustrating the outline of an image pickupapparatus to which the present embodiments may be applied. In FIG. 1, animage pickup apparatus 100 includes a pixel array 101 and a verticalselection circuit 102 that selects a row in the pixel array 101. In thepixel array 101, a plurality of photoelectric conversion units areprovided. The plurality of photoelectric conversion units may bearranged in two dimensions.

The vertical selection circuit 102 selects a certain row, and signalsare output from photoelectric conversion units included in the certainrow to vertical output lines. A vertical output line may be provided foreach column or for a plurality of columns, or a plurality of verticaloutput lines may be provided for each pixel column. When a plurality ofvertical output lines are provided for each pixel column, the speed atwhich signals are read may be increased.

A column circuit 103 receives signals read to the plurality of verticaloutput lines parallel to one another. The column circuit 103 may executeat least one of the following processes: amplification of the signals;analog-to-digital conversion; and noise reduction.

A horizontal selection circuit 104 sequentially, randomly, orsimultaneously selects the signals held by the column circuit 103 andoutputs the signals to a horizontal output line, which is notillustrated.

A serial interface 105 communicates with the outside in order to, forexample, determine an operation mode from the outside. It is to be notedthat the image pickup apparatus 100 may include, in addition to theillustrated components, for example, a timing generator, a controlcircuit, or the like that provides timing pulses for the verticalselection circuit 102, the horizontal selection circuit 104, and thecolumn circuit 103.

The block diagram of FIG. 1 may be applied to all the followingembodiments. In addition, the terms “vertical” and “horizontal” are usedfor the sake of convenience and therefore may be switched.

Next, FIGS. 14A and 14B illustrate examples of the equivalent circuit ofthe photoelectric conversion unit. FIG. 14A illustrates an example inwhich transistors having different functions are provided for eachphotoelectric conversion element. FIG. 14B illustrates an example inwhich transistors having different functions are provided for aplurality of photoelectric conversion elements in common.

Charge generated in photoelectric conversion elements 1401 a and 1401 bis transferred to input nodes of amplification transistors 1403 a and1403 b by transfer transistors 1402 a and 1402 b, respectively. Theinput nodes of the amplification transistors 1403 a and 1403 b may beconfigured by gates of the amplification transistors 1403 a and 1403 band a floating diffusion region electrically connected to the gates ofthe amplification transistors 1403 a and 1403 b. When pulses for turningon selection transistors 1404 a and 1404 b have been supplied to gatesof the selection transistors 1404 a and 1404 b, signals according to theinput nodes of the amplification transistors 1403 a and 1403 b areoutput to a vertical output line 1406. Thereafter, reset transistors1405 a and 1405 b set the voltage of the input nodes of theamplification transistors 1403 a and 1403 b to a certain voltage. Insuch a circuit configuration, by exclusively turning on the selectiontransistors 1404 a and 1404 b, signals of the photoelectric conversionelements 1401 a and 1401 b are read to a column circuit and processessuch as addition are performed, in order to realize capture of an imageand focus detection.

Next, FIG. 14B will be described. A basic operation is the same as inFIG. 14A. Charge generated in photoelectric conversion elements 1501 aand 1501 b are transferred to an input node of an amplificationtransistor 1503 by transfer transistors 1502 a and 1502 b, respectively.The input node of the amplification transistor 1503 may be configured bya gate of the amplification transistor 1503 and a floating diffusionregion electrically connected to the gate of the amplificationtransistor 1503. When a pulse for turning on a selection transistor 1504has been supplied to a gate of the selection transistor 1504, a signalaccording to the input node of the amplification transistor 1503 isoutput to a vertical output line 1506. Thereafter, a reset transistor1505 sets the voltage of the input node of the amplification transistor1503 to a certain voltage. In FIG. 14B, since the amplificationtransistor 1503 is shared by the plurality of photoelectric conversionelements 1501 a and 1501 b, the input node of the amplificationtransistor 1503 may perform addition. Therefore, added signals may beoutput to the vertical output line 1506 from the photoelectricconversion unit.

Specific embodiments will be described hereinafter. A term “impurityconcentration” used herein, in the claims, and in the drawings refers tonet impurity concentration compensated by impurities of an oppositeconductivity type. That is, it refers to so-called “net concentration”.A region in which the concentration of added p-type impurities is higherthan the concentration of added n-type impurities is a p-typesemiconductor region. On the other hand, a region in which theconcentration of added n-type impurities is higher than theconcentration of added p-type impurities is an n-type semiconductorregion.

First Embodiment

FIG. 2 is a schematic diagram illustrating upper surfaces ofphotoelectric conversion units 201 of an image pickup apparatus 100according to the present embodiment. A microlens 202 is provided foreach photoelectric conversion unit 201. In addition, each photoelectricconversion unit 201 includes a plurality of photoelectric conversionelements. In FIG. 2, each photoelectric conversion unit 201 includes twoPDs, namely a left PD 203 and a right PD 204, but the number of PDs maybe two or more. For example, four PDs or nine PDs may be included.Transfer gates 205 and 206 transfer charge generated in the PD 203 andthe PD 204, respectively, to a floating diffusion region 207. Althoughthe floating diffusion region 207 is shared by the two PDs 203 and 204in FIG. 2, a floating diffusion region may be provided for each PD,instead. Although the two photoelectric conversion units 201 areillustrated in FIG. 2, a larger number of photoelectric conversion unitsmay be provided, instead.

FIG. 3A is a diagram illustrating the cross-sectional structure ofphotoelectric conversion units according to the present embodiment, andFIG. 3B is a diagram schematically illustrating the potential ofsemiconductor regions taken along a broken line IIIB-IIIB illustrated inFIG. 3A relative signal carriers. FIGS. 3A and 3B are diagramsillustrating a case in which the OFD region is an n-type semiconductorsubstrate, and a so-called “OFD region in a vertical direction” (avertical OFD (VOFD) region) will be described with reference to FIGS. 3Aand 3B.

FIG. 3C is a diagram illustrating the cross-sectional structure ofphotoelectric conversion units according to the present embodiment, andFIG. 3D is a diagram schematically illustrating the potential ofsemiconductor regions taken along a broken line IIID-IIID illustrated inFIG. 3C relative to signal carriers. FIGS. 3C and 3D are diagramsillustrating a case in which the OFD region is an n-type floatingdiffusion region, a source region or a drain region of an n-type pixeltransistor, or the like. A so-called “OFD region in a lateral direction”(a lateral OFD (LOFD) region) will be described with reference to FIGS.3C and 3D. The image pickup apparatus 100 according to the presentembodiment may have the cross-sectional structures illustrated in FIGS.3A and 3C, or may have one of these cross-sectional structures.

First, the case of the VOFD region will be described. Color filters 301are included. Wires 302 drive transistors in pixels and supplies powersupply voltage and the like. A p-type semiconductor region 304 isprovided on an n-type semiconductor substrate 303. N-type semiconductorregions 203 and 204 are provided in such a way as to configure p-njunctions with the p-type semiconductor region 304. The n-typesemiconductor regions 203 and 204 are regions in which the potential islow relative to electrons, which are signal carriers, and the signalcarriers may be collected.

A combination between the p-type semiconductor region 304 and the n-typesemiconductor region 203 and a combination between the p-typesemiconductor region 304 and the n-type semiconductor region 204 eachform a PD. A left PD is a PD 203, and a right PD is a PD 204.

A p-type semiconductor region 305 is included in the adjacentphotoelectric conversion units and provided between adjacent two PDs. Ap-type semiconductor region 306 is provided between the PD 203 and thePD 204. The p-type semiconductor regions 305 and 306 may function aspotential barriers for signal carriers.

In the present embodiment, a relationship between the heights of thepotential barriers of the p-type semiconductor region 304 and the p-typesemiconductor region 306 is defined. More specifically, the p-typeimpurity concentration of the p-type semiconductor region 306 is madelower than the p-type impurity concentration of the p-type semiconductorregion 304. That is, a height 307 of the potential barrier generated bythe p-type semiconductor region 306 is made smaller than a height 309 ofthe potential barrier generated by the p-type semiconductor region 304.In doing so, when one of the PDs has been saturated, charge leaks intothe adjacent PD in the same photoelectric conversion unit before thecharge leaks into the VOFD region.

More preferably, a height 308 of the potential barrier generated by thep-type semiconductor region 305 is made larger than the height 307 ofthe potential barrier generated by the p-type semiconductor region 306.Even more preferably, the height 308 of the potential barrier generatedby the p-type semiconductor region 305 is made larger than the height309 of the potential barrier generated by the p-type semiconductorregion 304.

By using such structure, even if one of the PDs has been saturated dueto a difference in sensitivity between the PD 203 and the PD 204 or adifference in luminance between lays of light incident on the PD 203 andthe PD 204, respectively, a combined difference in sensitivity may bereduced.

As an example of preferable impurity concentration, the impurityconcentration of the p-type semiconductor region 304 is at least threetimes higher than the impurity concentration of the p-type semiconductorregion 306. More preferably, the impurity concentration of the p-typesemiconductor region 304 is at least ten times higher than the impurityconcentration of the p-type semiconductor region 306. It is to be notedthat the reason why a difference of at least three times higher is usedis that, on the basis of the potential of charge (about 26 mV at a roomtemperature of 27° C.), a difference between the potential barriers tothe same degree is supposed to be formed.

Furthermore, in addition to the above condition of impurityconcentration, the impurity concentration of the p-type semiconductorregion 305 may be higher than the impurity concentration of the p-typesemiconductor region 304. Furthermore, the impurity concentration of thep-type semiconductor region 305 is preferably at least three timeshigher than the impurity concentration of the p-type semiconductorregion 306. More preferably, the impurity concentration of the p-typesemiconductor region 305 is at least ten times higher than the impurityconcentration of the p-type semiconductor region 306. It is to be notedthat the reason why a difference of at least three times higher is usedis that, on the basis of the potential of charge (about 26 mV at a roomtemperature of 27° C.), a difference between the potential barriers tothe same degree is supposed to be formed.

The PDs and the combined output will be described with reference toFIGS. 4 and 5A to 5D. FIG. 4 illustrates a combined input/outputcharacteristic at a time when the input/output characteristics of thetwo PDs 203 and 204 and the outputs of the PDs 203 and 204 are combined.FIGS. 5A to 5D are diagrams schematically illustrating potentialstructure illustrated in FIG. 3B and generated charge. When light isincident on a PD, the PD performs photoelectric conversion, andelectron-hole pairs are generated. For the sake of explanation, FIG. 4illustrates a situation in which the PD 203 has higher sensitivity thanthe PD 204 or the amount of light incident on the PD 203 is larger thanthat incident on the PD 204. When the amount of light incident on thePDs 203 and 204 is within a range 401 illustrated in FIG. 4, the amountof charge generated is larger in the PD 203 than in the PD 204. Thiscondition is illustrated in FIG. 5A. A combined output of the PD 203 andthe PD 204 is indicating an appropriate value. Next, a situation inwhich the PD 203 has been saturated and the PD 204 has not beensaturated occurs in a range 402. At this time, as illustrated in FIG.5B, charge generated in the PD 203 may exceed the potential barrier 307and move to the PD 204. Therefore, in the range 402, the output of thePD 204 is an amount of charge obtained by combining the charge generatedin the PD 204 and the charge generated in the PD 203. Thus, by makingthe potential barrier 307 lower than the potential barrier 309, most ofthe charge generated in the PD 203 may leak into the PD 204 in the samephotoelectric conversion unit. More preferably, as illustrated in FIG.5B, the height of the potential barrier 308 is made larger than theheight of the power supply unit 307.

By making the potential barrier 307 lower than the potential barrier309, in the range 402, too, the combined output of the PD 203 and the PD204 may be close to an appropriate output as in the range 401. In arange 403, as illustrated in FIG. 5C, both the PDs 203 and 204 exceed asaturation level defined by the potential barrier 307, and the outputsof the PDs 203 and 204 increase up to a saturation level defined by thepotential barrier 308 or the potential barrier 309, whichever is lower.In FIGS. 3A to 3D or FIGS. 5A to 5D, since the potential barrier 309 islower than the potential barrier 308, the saturation level is defined bythe potential barrier 309. In a range 404, as illustrated in FIG. 5D,since the outputs of both the PDs 203 and 204 have increased up to thesaturation level defined by the potential barrier 309, the combinedoutput is also saturated.

In the above ranges, by making the height of the potential barrier 307smaller than the height of the potential barrier 309, the chargegenerated in the PD 203 may leak into the PD 204 in the samephotoelectric conversion unit. Although the height of the potentialbarrier 308 is made larger than the height of the potential barrier 309in the above description, a relationship between the potential barriers308 and 309 may be opposite. When the height of the potential barrier308 has been made larger than the height of the potential barrier 309,the saturation level of the PDs 203 and 204 is determined by thepotential barrier 308, and overflowing charge is discharged into thesemiconductor substrate 303. Thus, a phenomenon in which the overflowingcharge leaks into another PD and becomes an alias may be suppressed. Inaddition, when the height of the potential barrier 309 has been madelarger than the height of the potential barrier 308, the saturationlevel of the PDs 203 and 204 may be higher. In addition, the heights ofthe potential barriers 308 and 309 may be the same, or the height of thepotential barrier 308 may be slightly smaller than the height of thepotential barrier 309.

Next, a case in which the OFD region is an LOFD region will be describedwith reference to FIGS. 3C and 3D. An example in which a floatingdiffusion region is used as the LOFD region will be described.

A p-type semiconductor region 312 is provided between a floatingdiffusion region 311 and a PD 203. A p-type semiconductor region 312 mayfunction as a potential barrier for signal carriers. A transfer gate 310transfers electrons of electron-hole pairs generated in the PD 203 tothe floating diffusion region 311. The height of a potential barrier 307generated by the p-type semiconductor region 306 is made lower than theheight of a potential barrier 313 generated by the p-type semiconductorregion 312.

As illustrated in FIG. 3D, the height of the potential barrier 307 issmaller than the height of the potential barrier 313. By using suchstructure, even if either the PD 203 or a PD 204 has been saturated dueto a difference in sensitivity between the PDs 203 and 204 or adifference in luminance between lays of light incident on the PDs 203and 204, respectively, combined sensitivity may be close to constant.Furthermore, the height of the potential barrier 307 may be smaller thanthe height of the potential barrier 308.

As an example of preferable impurity concentration, the impurityconcentration of the p-type semiconductor region 312 that configures thepotential barrier 313 is at least three times higher than the impurityconcentration of the p-type semiconductor region 306 that configures thepotential barrier 307. More preferably, the impurity concentration ofthe p-type semiconductor region 312 is at least ten times higher thanthe impurity concentration of the p-type semiconductor region 306. It isto be noted that the reason why a difference of at least three timeshigher is used is that, on the basis of the potential of charge (about26 mV at a room temperature of 27° C.), a difference between thepotential barriers to the same degree is supposed to be formed.

Furthermore, in addition to the above condition of impurityconcentration, the impurity concentration of the p-type semiconductorregion 305 may be higher than the impurity concentration of the p-typesemiconductor region 312. Furthermore, the impurity concentration of thep-type semiconductor region 305 is preferably at least three timeshigher than the impurity concentration of the p-type semiconductorregion 306. More preferably, the impurity concentration of the p-typesemiconductor region 305 is at least ten times higher than the impurityconcentration of the p-type semiconductor region 306. It is to be notedthat the reason why a difference of at least three times higher is usedis that, on the basis of the potential of charge (about 26 mV at a roomtemperature of 27° C.), a difference between the potential barriers tothe same degree is supposed to be formed.

In the case of the LOFD region, too, the effects illustrated in FIGS. 4and 5A to 5D may be obtained. In addition, it is sufficient if the LOFDregion is provided in the lateral direction of an n-type semiconductorregion that configures a PD, and the LOFD region may be a source regionor a drain region of a transistor in the photoelectric conversion unit,or a dedicated LOFD region may be provided. The LOFD region isconfigured by an n-type semiconductor, and power supply voltage may besupplied to the LOFD region.

Either the VOFD region or the LOFD region may be used as a region intowhich the charge after saturation is discharged, but when the LOFDregion is used, the impurity concentration may be accurately controlledbecause the LOFD region is generally arranged at a region close to asurface and therefore may be formed by ion implantation at a shallowlevel.

In addition, if a potential control gate is provided above asemiconductor region between an OFD region and an n-type semiconductorregion that configures a PD, the conductivity type need not beparticularly limited. This is because the height of the potentialbarrier of the semiconductor region between the OFD region and then-type semiconductor region that configures the PD may be controlled byvoltage supplied to the potential control gate.

Second Embodiment

The present embodiment will be described with reference to the drawings.Components having the same functions as in the first embodiment aregiven the same reference numerals, and detailed description thereof isomitted. FIGS. 6A and 6B are diagrams illustrating the cross-sectionalstructure of photoelectric conversion units according to the presentembodiment. FIG. 6A schematically illustrates the cross-sectionalstructure of a portion taken along a dash-dot line VI-VI illustrated inFIG. 2, and FIG. 6B schematically illustrates the minimum potential of aportion taken along a broken line VIB-VIB illustrated in FIG. 6A.

A difference from the first embodiment is that a p-type semiconductorregion provided between photoelectric conversion elements included in asingle photoelectric conversion unit is configured by a first part whoseconcentration is low and second parts whose concentrations are higherthan the concentration of the first part. More specifically, the p-typesemiconductor region provided between PDs on which light collected by asingle microlens is incident is configured by a first part 601 whoseconcentration is low and second parts 602 whose concentrations arehigher than the concentration of the first part 601.

FIG. 6B illustrates a potential barrier 603 generated by the first part601. The height of the potential barrier 603 is lower than the height ofa potential barrier 605 generated by a p-type semiconductor region 304.In addition, the height of the potential barrier 603 may be lower thanthe height of a potential barrier 604 generated by a p-typesemiconductor region 305.

As an example of preferable impurity concentration, the impurityconcentration of the p-type semiconductor region 304 is at least threetimes higher than the impurity concentration of the first part 601. Morepreferably, the impurity concentration of the p-type semiconductorregion 304 is at least ten times higher than the impurity concentrationof the first part 601.

It is to be noted that although only one first part 601 is provided inthe present embodiment, a plurality of first parts 601 may be provided.In addition, with respect to depth, although a configuration is used inwhich the second parts 602, whose impurity concentrations are high, areprovided on and under the first part 601, the first part 601 need not besandwiched between the second parts 602 and may be provided on or underthe second parts 602.

Here, compared to the second parts 602, the first part 601 is likely tobe electrically connected to the PDs 203 and 204. When the PDs 203 and204 have been completely connected to each other electrically, theheight of the potential barrier 603 illustrated in FIG. 6B becomessmaller, and it becomes difficult to distinguish signals of the PD 203and signals of the PD 204 during reading. That is, the accuracy of focusdetection might decrease. In such a case, the first part 601 may beprovided at a position whose depth from a surface of the substrate isdifferent from the depths of portions 203′ and 204′ (hereinafterreferred to as impurity concentration peak positions) of n-typesemiconductor regions of the PDs 203 and 204, respectively, in which then-type impurity concentrations are the highest. By making the depth ofthe first part 601 and the depths of the impurity concentration peakpositions 203′ and 204′ different from each other, the independence ofthe signals of the PDs 203 and 204 may be certainly assured, and theheights of barriers of the PD 203 and the PD 204 may be smaller.Therefore, the knee characteristic may be suppressed and an output rangein which independence is maintained may be increased.

Although a case of the VOFD region has been described in the presentembodiment, the same holds true for a case of the LOFD region.

Third Embodiment

A third embodiment will be described with reference to the drawings.FIG. 7 is a diagram illustrating the planar structure of pixelsaccording to the present embodiment viewed from a light incident side.Components having the same functions as in the first and secondembodiments are given the same reference numerals, and detaileddescription thereof is omitted. A difference of the present embodimentfrom the second embodiment is that a first part 701 and a second part702 are provided at different positions between n-type semiconductorregions that configure adjacent PDs in the same photoelectric conversionunit when viewed in plan. With respect to other configurations, the sameconfigurations as in the first and second embodiments may be used.

In FIG. 7, a p-type semiconductor region provided between photoelectricconversion elements PD1 and PD2 included in a single photoelectricconversion unit is configured by including the first part 701 and thesecond part 702 whose p-type impurity concentration is higher than thatof the first part 701. In addition, when the OFD region is a VOFDregion, the p-type impurity concentration of the first part 701 is lowerthan the p-type impurity concentration of a p-type semiconductor region304. When the OFD region is an LOFD region, the p-type impurityconcentration of the first part 701 is lower than the p-type impurityconcentration of a p-type semiconductor region 312.

As illustrated in FIG. 7, p-type impurity concentration may be differentat different positions in a plane. The cross-sectional structure of aportion taken along a broken line IIIA-IIIA illustrated in FIG. 7 is thesame as the cross-sectional structure illustrated in FIG. 3A. In FIG. 7,unlike the structure illustrated in FIG. 6A, the impurity concentrationsof isolation regions between the PDs are different not in asemiconductor substrate direction but in the plane.

Here, the state of the potential of the first part 701 is likely tochange depending on the number of electrons existing in the PDs 203 and204. Therefore, the probability of whether charge generated in the firstpart 701 moves to the PD 203 or the PD 204 varies between a stateimmediately after the beginning of accumulation in the PD 203 and the PD204 and a state in which a larger amount of charge exists in one of thePDs 203 and 204, that is, for example, the PD 203. For example, when alarger amount of charge exists in the PD 203 immediately after thebeginning of the accumulation, the potential of the first part 701changes due to Coulomb interaction of the charge existing in the PD 203.Thereafter, the probability that the charge generated in the first part701 moves to the PD 204 increases. That is, the probabilities thatcharge moves to the PD 203 and to the PD 204 change. In such structure,for example, feedback that cancels a difference in signals between thePDs 203 and 204 for detecting a difference in phase may be produced,thereby reducing the accuracy of the focus detection.

Such reduction in accuracy may be suppressed by adopting the followingarrangement of the first part 701. More specifically, when the firstpart 701 is viewed in plan, the first part 701 is arranged at a positiondeviated from a projection position of a center position of eachmicrolens onto light receiving surfaces of the photoelectric conversionelements. In FIG. 2, the projection positions are on a line that crossessubstantially the centers of the microlenses, and the first part 701 isarranged in such a way as to be offset in an upper direction of thefigure when viewed in plan. The essence of this arrangement is toisolate the first part 701 from regions around positions at which theintensity of light collected by each microlens is the highest. Forexample, in FIG. 7, the first part 701 is offset in the upper direction,but the direction of offset is not limited to this, and the directionmay be a lower direction or, depending on the case, a left or rightdirection. As a preferable amount of offset, the first part 701 isarranged at a position 0.1 micrometer away from the center of themicrolens. More preferably, the first part 701 is arranged at a position0.2 micrometer away from the center of the microlens. This holds truefor a case in which the range of wavelengths used by the PDs is aso-called visible light range. That is a case in which the wavelengthsof visible light are about 0.4 to 0.8 micrometer and the focus of themicrolens exists on the photoelectric conversion elements. In addition,because the light focus state of the photoelectric conversion elementsalso depends on the F value of an objective lens, a larger value may beused as the amount of offset at a time when a photoelectric conversionapparatus according to the present embodiment is applied to an opticalsystem whose F value is small. For example, in the case of a system thatmay set the F value of the objective lens to 2.0, light having a maximuminclination of 14° relative to the perpendicular is incident on themicrolens. When the distances between the microlens and the PDs are 2μm, the focal position of light incident at an inclination of 14° isoffset, by 0.5 μm at least in one direction, from positions of the lightreceiving surfaces onto which the center position of the microlens isprojected. In this case, it is particularly effective to arrange thefirst part 701 in such a way as to be offset from the center position ofthe microlens by 0.5 μm or more. In addition, the upper limit of theamount of offset is half the pitch of the adjacent photoelectricconversion elements.

An effect produced by the present embodiment in addition to the effectsproduced by the first and second embodiments is that the potentialbarriers may be adjusted using a layout pattern. Therefore, the degreeof freedom in designing increases compared to a case in which thepotential barriers are designed only by adjusting the depths ofimplantation of impurity ions in p-type semiconductor regions and theimpurity concentrations. In addition, when the potential barriers are tobe set in accordance with the first and second embodiments, a pluralityof levels, namely the depths of implantation of impurity ions,concentrations, and heat treatment, need to be provided before theimpurity ions are implanted in the p-type semiconductor regions.However, according to the present embodiment, a level may be provided inthe layout pattern of a photomask used in a process for defining regionsin which impurity ions are to be implanted. Therefore, by evaluating asingle trial wafer, a layout pattern that may form appropriate potentialbarriers may be selected from a plurality of layout patterns.

Fourth Embodiment

A fourth embodiment will be described to the drawings. Components havingthe same functions as in the first to third embodiments are given thesame reference numerals, and detailed description thereof is omitted. Adifference of the present embodiment from the first to third embodimentsis that a p-type semiconductor region provided between n-typesemiconductor regions 203 and 204 includes a first part 801 and secondparts 802 and the width of the first part 801 is smaller than the widthsof the second parts 802.

FIG. 8 is a diagram illustrating the cross-sectional structure of pixelsaccording to the present embodiment. Although the first part 801 and thesecond parts 802 are provided at different depths in FIG. 8, the firstpart 801 and the second parts 802 may be provided at the same depth anddifferent positions when viewed in plan as described in the thirdembodiment. In addition, the first part 801 may be configured by aplurality of portions that are isolated from one another.

In the above embodiments, a case in which potential barriers areconfigured by using n-type semiconductor regions that may accumulatesignal carriers and p-type semiconductor regions of the oppositeconductivity type has been described. However, insulator separation maybe used in combination with this. Alternatively, a configuration may beused in which the height of a potential barrier between an OFD regionand a PD region is controlled by using a control gate. In this case, theabove-described relationship between the heights of the potentialbarriers needs to be satisfied while PDs are accumulating signalcarriers.

Application to Focus Detection Apparatus

The photoelectric conversion apparatuses described in the aboveembodiments may be used as image pickup apparatuses and apparatuses thatperform focus detection in image pickup surfaces. An example in whichfocus detection in an image pickup surface is performed using phasedifference detection during capture of an image will be specificallydescribed.

In the following description, FIGS. 9, 10A, and 10B will be referred to.FIG. 9 is a conceptual diagram illustrating a situation in which a lightbeam emitted from an exit pupil of an imaging lens is incident on animage pickup apparatus 901. Microlenses 202, color filters 301, and aplurality of photoelectric conversion elements PD1 and PD2 on whichlight collected by one of the microlenses 202 is incident areillustrated. An exit pupil 902 of the imaging lens is also illustrated.Here, the center of a light beam emitted from the exit pupil 902 to aphotoelectric conversion unit including the microlenses 202 will bereferred to an optical axis 903. The light emitted from the exit pupil902 is incident on the image pickup apparatus 901 along the optical axis903 as the center thereof. Light beams 906 and 907 are light beams alongedges of light that passes through a certain region 904 of the exitpupil 902, and light beams 908 and 909 are light beams along edges oflight that passes through a certain region 905 of the exit pupil 902. Asmay be seen from FIG. 9, among light beams emitted from the exit pupil902, light beams above the optical axis 903 are incident on an upper PD,and light beams below the optical axis 903 are incident on a lower PD.That is, these PDs are receiving light beams from different regions ofthe exit pupil 902 of the imaging lens.

By utilizing this characteristic, detection of a phase difference isperformed. When an image pickup region in a region within a pixel isviewed in plan, data obtained from one PD will be referred to as a firstline, and data obtained from the other OD will be referred to as asecond line in a plurality of photoelectric conversion elements on whichlight collected by a single microlens is incident. Thereafter, byobtaining correlation data between the lines, phases may be detected.

For example, in FIG. 9, data obtained from the lower PD 203 will bereferred to as the first line, and data obtained from the upper PD 204will be referred to as the second line in the photoelectric conversionelements on which light collected by one of the microlenses 202 isincident. In this case, the photoelectric conversion element PD1 outputsdata regarding one pixel from the first line data, and the photoelectricconversion element PD2 outputs data regarding one pixel from the secondline data. FIGS. 10A and 10B illustrate line data at a time when animage is formed using a point-source light. FIG. 10A illustrates firstline data and second line data at a time when the image is in focus. Thehorizontal axis represents the positions of the pixels, and the verticalaxis represents output. When the image is in focus, the first line andthe second line overlap. FIG. 10B illustrates line data at a time whenthe image is out of focus. At this time, the first line and the secondline have a phase difference, and the positions of the pixels aredifferent. By calculating this amount of deviation 1001, how largely theimage is out of focus may be identified. The image may be in focus bydetecting the phases using such a method and by driving the lens.

Next, generation of image data in these pixel arrangements will bedescribed. As described above, the focus may be detected by separatelyreading the signals of the photoelectric conversion elements PD1 and PD2from the image pickup apparatus 901 and by performing calculation fordetecting the phase difference. In addition, by adding the signals ofthe PDs 203 and 204 on which the light collected by one of themicrolenses 202 is incident, a captured image may be generated.

However, in a state in which one of the PDs 203 and 204 has beensaturated, that is, in a state illustrated in FIG. 5B, 5C, or 5D, thesignals of the PDs 203 and 204 are different from the outputs separatelyobtained by the PDs 203 and 204. Therefore, the signals of the PDs 203and 204 might be judged to have low reliability. In such a case, asequence in which the phase detection is not performed or the phasedetection is stopped may be adopted. That is, the image pickup apparatus901 may be operated while judging whether or not to perform the phasedifference detection on an image plane of the image pickup apparatus 901in accordance with the signals of the PDs 203 and 204 or charge that maybe accumulated.

Although a pixel around the center of the image pickup apparatus 901 hasbeen described with reference to FIG. 9, the accuracy may be furtherimproved when a pixel for focus detection is provided not at the centerof an image pickup region but at an edge of the image pickup regionbecause the difference in the amount of incident light between PDs islarger in a pixel at a periphery of the image pickup apparatus 901.

Application to Image Pickup System

FIG. 11 illustrates an example of an image pickup system to which theimage pickup apparatus according to each of the above embodiments may beapplied. In FIG. 11, a lens unit 1101 forms an optical image of asubject onto an image pickup element 1105, and a lens driving apparatus1102 performs zoom control, focus control, aperture control, and thelike. A shutter 1103 is controlled by a shutter driving apparatus 1104.The image pickup element 1105 obtains the image of the subject formed bythe lens unit 1101 as an image signal, and an image pickup signalprocessing circuit 1106 performs various types of correction on theimage signal output from the image pickup element 1105 and compressesdata. A timing generation unit 1107 is driving means that outputsvarious timing signals to the image pickup element 1105 and the imagepickup signal processing circuit 1106. An overall control arithmeticunit 1109 performs various types of calculation and controls theentirety of an image pickup apparatus, a memory unit 1108 temporarilystores image data, and a recording medium control interface unit 1110records image data on or reads image data from a recording medium. Arecording medium 1111 is a removable recording medium such as asemiconductor memory on which image data is recorded or from which imagedata is read, and an external interface unit 1112 is an externalinterface.

Next, the operation of a digital camera when an image is captured usingthe above configuration will be described. When a main power supply hasbeen turned on, the power of a control system is turned on, and thepower of an image pickup circuit such as the image pickup signalprocessing circuit 1106 is also turned on.

When a release button (not illustrated) has been pressed, rangingcalculation is performed on the basis of data from the image pickupelement 1105 and the overall control arithmetic unit 1109 performscalculation of a distance to the subject on the basis of a result of theranging calculation. Thereafter, the lens driving apparatus 1102 drivesthe lens unit 1101 to judge whether or not an image is in focus. Thus,the digital camera using the image pickup apparatus according to each ofthe present embodiments may perform a focusing operation at high speedwhile suppressing generation of a false color.

The embodiments may be applied to various types of photoelectricconversion elements, and it is especially effective in a configurationin which there is a difference in sensitivity or the amount of incidentlight between photoelectric conversion elements. For example, in theabove embodiments, an example in which a pixel is used for focusdetection of an objective lens has been described. However, acharacteristic of the image pickup apparatus is to secure the linearityof an output at a time when reading in which signals of a plurality ofphotoelectric conversion elements are added is performed, and thereforean application other than the focus detection is possible. For example,the plurality of photoelectric conversion elements may be provided withtwo types of color filters whose pass bands are different from eachother, instead. More specifically, R′, G′, and B′ color filters whosepass bands are different from one another are provided for colors of R,G, and B, respectively. When these color filters have been separatelyread, signals of six colors may be obtained, thereby improving colorreproducibility. On the other hand, when addition of PDs and anoperation for obtaining signals of three colors, namely R+R′, G+G′, andB+B′, have been performed, sensitivity may be increased, therebyimproving the signal-to-noise (S/N) ratio. The embodiments may beapplied to a configuration in which it is possible to switch betweenthese two capture modes in the same image pickup apparatus.

In addition, although a configuration in which electrons are used assignal carriers has been described, holes may be used, instead. In thiscase, the opposite conductivity type is used as the conductivity type ofeach semiconductor region.

While the disclosure has been described with reference to exemplaryembodiments, it is to be understood that the disclosure is not limitedto the disclosed exemplary embodiments. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No.2012-008204 filed Jan. 18, 2012, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A photoelectric conversion apparatus comprising:a plurality of photoelectric conversion units, each including aplurality of photoelectric conversion elements, wherein signal carriersgenerated in the plurality of photoelectric conversion elements includedin the plurality of photoelectric conversion units or signals based onthe signal carriers are added, wherein each of the plurality ofphotoelectric conversion elements includes a first semiconductor regionof a first conductivity type that collects the signal carriers, whereinthe first semiconductor regions included in photoelectric conversionelements that are included in each of the photoelectric conversion unitsand that are arranged adjacent to each other sandwich a secondsemiconductor region of a second conductivity type, and wherein a heightof a potential barrier for the signal carriers generated in a certainregion of the second semiconductor region is smaller than a height of apotential barrier for the signal carriers generated in a region betweeneach of the first semiconductor regions and an overflow drain region ofthe first conductivity type.
 2. The photoelectric conversion apparatusaccording to claim 1, wherein the region between each of the firstsemiconductor regions and the overflow drain region of the firstconductivity type is a third semiconductor region of the secondconductivity type.
 3. The photoelectric conversion apparatus accordingto claim 1, wherein each of the photoelectric conversion units includesa plurality of photoelectric conversion elements on which lightcollected by a single microlens is incident.
 4. The photoelectricconversion apparatus according to claim 1, wherein the overflow drainregion is a semiconductor region of the first conductivity type arrangedin a vertical direction of the first semiconductor region.
 5. Thephotoelectric conversion apparatus according to claim 4, wherein thesemiconductor region of the first conductivity type is a semiconductorsubstrate of the first conductivity type.
 6. The photoelectricconversion apparatus according to claim 1, wherein the overflow drainregion is a semiconductor region of the first conductivity type arrangedin a lateral direction of the first semiconductor region.
 7. Thephotoelectric conversion apparatus according to claim 6, wherein thesemiconductor region of the first conductivity type is a source regionor a drain region of a transistor of the first conductivity typeincluded in each of the photoelectric conversion units.
 8. Thephotoelectric conversion apparatus according to claim 2, whereinimpurity concentration of the third semiconductor region is at leastthree times higher than impurity concentration of the certain region. 9.The photoelectric conversion apparatus according to claim 8, wherein theimpurity concentration of the third semiconductor region is at least tentimes higher than the impurity concentration of the certain region. 10.The photoelectric conversion apparatus according to claim 1, wherein thesecond semiconductor region includes a first part and a second part, andimpurity concentration of the first part is lower than impurityconcentration of the second part or a width of the first part whenviewed in plan is smaller than a width of the second part when viewed inplan.
 11. The photoelectric conversion apparatus according to claim 10,wherein the first part is arranged at a depth different from a depth atwhich the second part is arranged.
 12. The photoelectric conversionapparatus according to claim 10, wherein, when the second semiconductorregion is viewed in plan, the first part is arranged at a positiondifferent from a position at which the second part is arranged.
 13. Thephotoelectric conversion apparatus according to claim 10, wherein adepth of an impurity concentration peak of the first part is differentfrom a depth of an impurity concentration peak of the firstsemiconductor region.
 14. The photoelectric conversion apparatusaccording to claim 10, wherein each of the photoelectric conversionunits includes a plurality of photoelectric conversion elements on whichlight collected by a single microlens is incident, and wherein the firstpart is arranged in such a way as to be offset at least in one directionrelative to a projection position of a center position of the microlensonto a light receiving surface.
 15. The photoelectric conversionapparatus according to claim 14, wherein an amount of offset is 0.1 μmor more.
 16. The photoelectric conversion apparatus according to claim1, wherein the plurality of photoelectric conversion elements arearranged at different positions when viewed in plan.
 17. Thephotoelectric conversion apparatus according to claim 1, wherein afourth semiconductor region of the second conductivity type is providedbetween the first semiconductor regions included in adjacentphotoelectric conversion elements included in different photoelectricconversion units arranged adjacent to each other, and a height of apotential barrier generated at least in a certain region of the secondsemiconductor region is smaller than a height of a potential barriergenerated in the fourth semiconductor region.
 18. The photoelectricconversion apparatus according to claim 17, wherein the height of thepotential barrier generated in the fourth semiconductor region is largerthan a height of a potential barrier generated in the thirdsemiconductor region.
 19. An image pickup system comprising: thephotoelectric conversion apparatus according to claim 1, wherein captureof an image is performed using signals obtained by adding signals of theplurality of photoelectric conversion elements included in the pluralityof photoelectric conversion units, and wherein focus detection duringthe capture of the image is performed using at least one of the signalsof the plurality of photoelectric conversion elements included in theplurality of photoelectric conversion units.
 20. The image pickup systemaccording to claim 19, wherein the focus detection is stopped when anamount of charge that is accumulated by one or more photoelectricconversion elements in the plurality of photoelectric conversion unitshas been exceeded.